Download C 2 = C 1 + h

Lecture 1-2 continued
Material balance and properties
Uplift and subsidence.
Topography, crustal and lithospheric thicknesses,
1) LATERAL TRANSPORT OF MATERIAL (tectonic extrusion)
2) VERTICAL TRANSPORT OF MATERIAL (fundamental change in
physical properties (SUBDUCTION AND EDUCTION)
AIRY ISOSTASY:
Thicker crust in orogenic belts gives higher topography because m > c
(1) C2 = C1 + h (m/(m - c)
h = (C2 - C1)(m - c)/ m
A thick, light crust floats high.
What happens if the crust and/or mantle density change? Example:
Partial eclogitization of orogenic crust, (100% below C2n km)
(2) C2 = C2n + [C1 (c - m) - m h + C2n (m - c)] / (e- m)
= C2n + [C2n - C1) (m - c) - m h ] / (e - m)
h C1 C2 C2n -
Elevation (above sea level)
Normal crust thickness (≈ 30 km)
Orogenic crust thickness
Orogenic crust without eclogitization
Densities of (m) mantle, (c) crust and (e) eclogitized crust
Metamorphic reactions change mineral assemblages:
•
New minerals => different density, rheology and
petrophysical properties
Dilation related to
1) Gabbro
=> eclogite transition is
≈ - 15 %
2) Amphibolite => eclogite transition is
≈ - 18 %
3) Peridotite => serpentinite transition is ≈ + 35 %
•
Metamorphic reactions break down minerals and so
may enhanced deformation
(increased strain / strain-rate)
From Hacker 2004
Some density
measurements of
rocks with near
identical geochemical
compositons
from the Bergen area
What are the implications for the
topography in Mountain Belts?
Crustal thickening => uplift
Mantle lithosphere thickening => subsidence
Lets look at some examples of modelling
where the petrophysical changes related to
metamorphic reactions and their reactions
rates have been considered.
MODELLED TOPOGRAPHIC EVOLUTION RELATED TO
THICKENING AND THINNING OF LITHOSPHERIC MANTLE
AND CRUST
Lithosphere
Delamination
Lithospheric thickening
a) No eclogitization
b) Eclogitization
half-time 6.4 myr
c) Eclogitization
half-time 3 myr
Tectonic denudation
a) No amphibolitization
b) amphibolitization
half-time 3 myr
c) No tectonic denudation
After Dewey et al. 1993
The petrophysical effect of
metamorphism.
Density changes related to
equilibrated prograde
metamorphism of hydrated
oceanic mantle lithosphere.
Notice that this diagram is
Particularly relevant for
Benioff zones.
(after Hacker et al. 2003)
From Hacker 2004
Hacker et al. (2002)
Blueschist
facies
earthquakes
Example, Japan
INTER-PLATE
EARTHQUAKES
INTRA-PLATE
EARTHQUAKES
Example, Costa Rica
The petrophysical effect of
metamorphism.
Density changes related to
equilibrated prograde
metamorphism of crust with,
density
Granitic
2.74 g/cm3
Andesitic
2.84 g/cm3
Gabbroic
2.95 g/cm3
compositions.
(Calculated by Henry et al. 2001)
(Henry et al. 2001)
Here we assumed that most
of the subducted crust reacted
and achieved mantle-type density.
We were mostly concerned with
keeping the topography realistic.
The key element is the
mantle-wedge above the
subducted part of the Continent.
METAMORPHISM, DENSITY STRUCTURE and TOPOGRAPHY
(from unpublished thesis by M. Krabbendam 1998)
AIRY ISOSTASY:
C2 = C1 + h (m/(m - c)
C1 - Normal crust thickness (≈ 30 km)
C2 - Orogenic crust thickness (≈ 30 km)
h - Elevation (above sealevel)
 - Densities of (m) mantle and (c) crust
METAMORPHISM,
DENSITY STRUCTURE
and TOPOGRAPHY
(from Krabbendam 1998)
AIRY ISOSTASY:
C2 = C1 + h (m/(m - c)
C1 - Normal crust thickness (≈ 30 km)
C2 - Orogenic crust thickness (≈ 30 km)
h - Elevation (above sealevel)
 - Densities of (m) mantle and (c) crust
METAMORPHISM AND MODELLED DENSITY
STRUCTURE IN THE ALPS
CONVERGENCE RATES 8 (TOP) AND 4 MM/YR,( after Henry et al 2001)
MODELLED (DOTTED) AND OBSERVED (SHADED)
TOPOGRAPHY OF THE ALPS (Henry et al 2001)
IMPORTANT DISTINCTION BETWEEN CONSEPTS!
UPLIFT VS. SUBSIDENCE
UPLIFT
=> SURFACE IS RAISED RELATIVE TO
REFERENCE
SUBSIDENCE => SURFACE IS LOWERED RELATIVE TO
REFERENCE
EXHUMATION VS. BURIAL
EXHUMATION => ROCKS APPROACH THE SURFACE
BURIAL
=> ROCKS MOVE AWAY FOR THE SURFACE
(IRRESPECTIVE OF UPLIFT OR SUBSIDENCE)
Some important points brought out by the first lectures:
Pro- and retrograde metamorphic reactions play important
roles for the dynamics in orogenic belts in that they give
• Changes in petrophysical properties
(density structure and hence evolution of topography)
• Reaction enhanced deformation
(increased strain (strain-rate) in zones of reaction)
• Material balance and cross-sections, which in turn is used to
estimate shortening